1. Field of the Invention
The present invention relates to a moving image signal encoding apparatus for performing compression encoding of a moving image signal for transmission of the moving image signal or recording of the same on a recording medium and a decoding apparatus for decoding the codes which have been transmitted or reproduced from a recording medium to obtain a reproduced image.
2. Description of Prior Art
Recently, in the moving image signal encoding apparatus and decoding apparatus, in consequence of the developments of television telephones and television conference systems, various compression encoding systems have been in practical use. As a procedure for curtailing the information amount to be used in these encoding systems there is a frame decimation. This is to curtail the information amount by decimating selected frames of the moving image signal in encoding. Since the movements of the reproduced images become unnatural by decimating the frames, frame interpolation is carried out in the decoding apparatus to obtain the reproduced images having smooth movements.
As one of the precedents to carry out frame interpolation with a moving image signal decoding apparatus there is a construction shown in U.S. Pat. No. 4,727,422. Hereinafter, the construction of the conventional moving image signal encoding apparatus and decoding apparatus is described.
The input of the moving image signal encoding apparatus is a moving image signal of 30 frames/sec. The inputted moving image signal, after decimation of the number of frames to 1/2, becomes a moving image signal of 15 frames/sec. These remaining frames of the moving image signal will be encoded. In the description hereinafter, these frames are called "encoded frames". Inter-frame motion vectors are obtained from the encoded frames. The motion vectors are obtained on a block by block basis. The motion vectors are used for frame interpolation in the decoding apparatus. The encoded frames and the motion vectors are respectively encoded, after which additional information is incorporated to obtain an output signal of the moving image signal encoding apparatus. The output signal is sent out to a transmission channel or recorded on a recording medium.
The moving image signal decoding apparatus is to decode the signal encoded by the moving image signal encoding apparatus and reproduce the moving image signal. By a signal receiving circuit, each code is received from the transmission channel or read out from the recording medium. The codes are decoded by respective decoding circuits to become the reproduced frames and the motion vectors. The frequency of the reproduced frames is 15 frames/sec. A frame interpolation circuit obtains interpolated frames each positioned between two reproduced frames. The frame interpolation is a motion compensating frame interpolation using a motion vector between the frames. By alternately outputting the interpolated frames and the reproduced frames, an output image signal of 30 frames/sec is obtained.
However, the above construction involves the problem to cause errors to the interpolated frames because there are no correct motion vectors in such cases that: (1) there are objects which move in different directions from each other in a block; (2) the background appears from the shade of a moving object or the background is hidden by a moving object; (3) the moving object changes in shape; and (4) there is a movement accompanied with rotation.